1. Field of the Invention
This invention relates to a drill or drill bit, particularly a rotary carbide drill, especially adapted for drilling holes in laminates and other composite structural materials which contain aramid fibers or fabrics comprising aramid fibers and to methods of making such drills.
2. Description of the Prior Art
Aramid or aromatic amide polymer fibers and fabrics made from such fibers are being advantageously employed in composite materials in place of glass fibers. Glass fiber laminates, e.g., are made from a plurality of woven glass fabric reinforcing sheets impregnated and bonded together by organic resins such as epoxy resins. Such laminates have desirable physical properties. The laminates and other composite materials are suitable for various structures and are widely used as substrates for printed circuits. The composite materials must sometimes be provided with accurately sized, clean holes with essentially no projecting fibers or fiber portions from the surface of the hole. The high strength of the inorganic glass fibers and the abrasiveness of the glass fibers make it difficult to provide such holes. Rotary carbide drills, e.g. tungsten carbide or titanium carbide, have effectively provided such holes.
New organic fibers of aramid (aromatic amide) have a combination of outstanding physical properties even superior to glass, exhibiting tensile strengths in excess of 300,000 psi, a high modulus and relatively low specific gravity. Composites comprising aramid fibers appear to have application in aircraft and aerospace structures as well as in substrates for printed circuits, particularly in resin bonded aramid fiber laminates similar to the glass fiber laminates described above. However, it appears that some of the properties which provide the aramid fiber composites with desirable physical advantages over the glass fiber composites, create other difficulties. The machining of aramid fiber composites is difficult compared even to the already difficult to machine glass fiber composites. This is particularly true when drilling holes. The high strength aramid fibers are difficult to shear or cut and the problem is apparently compounded by the softer resin binder, into which fibers may be forced and thus escape clean, uniform shearing or cutting. In any event, it is known that holes drilled in resin bonded laminates of aramid fiber reinforcing sheets or other aramid composites, are too frequently fuzzy or frayed. Such holes are particularly unacceptable in printed circuit substrates where the hole surfaces are metal plated to provide a conductive path from a metal layer on one side of the substrate to a metal layer on the other side of the substrate.
While it may be necessary and, indeed, known to employ carbide materials for the drills for aramid composites to provide an acceptably hard and abrasion resistant tool, the hardness makes it difficult to generate complex geometries in such drills. The drill points, flutes, cutting edges, relief angles, etc. must generally be generated in such hard materials with even harder diamond cutting tools. It should be apparent that simple geometries are particularly advantageous in drills of such materials because they are more easily generated.